With the development of industrialization and economy, the application of high-purity oxygen and nitrogen is becoming increasingly wider, including fields such as oil refining, medical treatment, metal production, food processing and chemicals. As two main techniques for preparing high-purity oxygen and nitrogen, separation of air by cryogenic distillation and pressure swing adsorption (PSA) are already known. As adsorbent for oxygen production by means of separation of air by PSA, LiLSX sieves have the advantages of high nitrogen adsorption capacity and high nitrogen-oxygen separation coefficient. The key to preparation of LiLSX molecular sieves as adsorbent is the exchange of NaLSX molecular sieves with Li+. Research indicates that there is almost no change in adsorption of molecular sieves for N2 when the exchange degree of Li+ is lower than 70%, and that the adsorption of molecular sieves for N2 linearly increases only when the exchange degree of Li+ increases to 100% from 70%. Therefore, a large amount of Li+ is needed for the preparation of LiLSX molecular sieves. Furthermore, due to the rapid development of lithium-ion batteries and the gradual reduction of lithium reserves, the price of lithium has been rising since 2016. As a result, the production of LiLSX molecular sieves is quite costly.
The separation of air by molecular sieves as adsorbent is based on the interaction between the electric field gradient of cations of molecular sieves and the quadrupole moment of N2; and this interaction has a lot to do with the sites of cations of molecular sieves. FIG. 1 shows the sites of cations of faujasite. For LiX molecular sieves, only cations at the site III (SIII) can be interacted with N2 (O2), because Li+ at site SII cannot be effectively utilized due to the electronic shield effect of oxygen and Li+ of small size around the skeleton.
It is generally necessary to pre-heat molecular sieves for dehydration, before use. Usually, this process will last for at least 4 hours at 400° C. In this case, migration or diffusion of cations to their equilibrium sites will occur. For molecular sieves with mixed cations, it has been found by research that cations of low mass tend to migrate to lower sites (SI, SI′, SII, SII′). For example, in CeNaCa—X molecular sieves with mixed cations, Na and Ca are at lower sites while Ce(III) cations are at some exosites. In NaSr—X zeolites with mixed canons, Na+ is at site SI while Sr2+ is at site SII. Such distribution of cations also exists in the skeleton of LiNa-LSX molecular sieves.
In the field of oxygen production by means of separation of air by PSA/VPSA, Li-LSX molecular sieves are highly favored. In such application fields, some other molecular sieves are also used, especially LSX molecular sieves with alkali metal cations. Chao et al. have found that A-type and X-type zeolites with mixed cations, including Li+ (30% to 90%) and alkali metal cations (10% to 70%), for example. Ca2+ and Sr2+ have high adsorption for N2. In these zeolites with mixed cations, at least 70 mol % of Li+ is generally needed (the mole percentage of alkali metal ions is less than 30 mol %), and it is usually to exchange with Li+ first and then with alkali metal ions. The present invention is aimed at synthesizing molecular sieve adsorbents which contain cheap or rich alkali metal cations as dominant elements and a small amount of Li+ and can be used for gas separation.